Quasi-isothermal temperature-modulated differential scanning calorimetry was employed in a study of thermoreversible structural changes in the melting range of semicrystalline polymers. The results indicated a reversible melting and crystallization process occurring at the fold surfaces of crystallites of polyethylene and poly(ethylene oxide). For polyethylene, good agreements were found with reported small-angle X-ray scattering data and Fischer＇s theory. Surface melting and crystallization depend on the ability of the chains in the crystals to carry out a sliding diffusion. This was shown by a comparison of polyethylene and poly(ethylene oxide) with other polymers such as poly(ethylene terephthalate), polycaprolactone, isotactic polypropylene, and syndiotactic polypropylene. When the longitudinal chain mobility in the crystals is much reduced or completely absent, only a small or no excess reversing heat capacity is observed. The special performance of short-chain poly(ethylene oxide) is indicative for the metastability of the crystals built of once-folded chains.